1. Field of the Invention
This invention relates to a thermosetting elastomeric composition and a process for its production. More particularly, this invention is concerned with an injection-moldable elastomeric composition comprising a mixture of a crystalline interpolymer comprising ethylene and propylene; a low density polyethylene; one or more multifunctional vinylic or allylic monomers which copolymerizes in the presence of peroxide and which functions as a co-curing agent, an amount of carbon sufficient to render the composition, when crosslinked, electrostatically conductive; and a peroxide crosslinking agent. Most particularly, this invention relates to the said composition, a method for its production and to cross-linked structures made from the composition which are fully cured and thermoset.
Recently the need for improved fuel economy in the operation of automobiles has led to smaller cars and automotive manufacturers lowering the weight of automobiles by replacing steel, particularly in the body, with light metal alloys and polymeric compositions. Currently automotive companies are developing elastomeric structures (fascia) for the front and rear ends of automobiles to replace the present steel fender extensions, radiator grilles and the like. These structures are required to flex on impact in concert with energy absorbing devices and return, undamaged, to their original shape when the distorting forces are released. In addition, the structure must readily accept paint and the finished surface must have adequate weather resistance and show a minimum of marking or marring on impact and recovery.
Suitable and commercially acceptable compositions for the fabrication of these structures must not only be capable of being amenable to mass production methods, as by conventional injection molding techniques, but the finished product must possess the necessary physical properties of high flexural stiffness, high tensile strength, hardness, ability to recover rapidly to its original state when deformed and to be mar and tear resistant. In addition, since fascia structures are usually painted electrostatically and the paint cured in an oven, the structure must be electrically conductive and must not undergo deformation when passing through the paint oven.
2. Prior Art
Currently two types of polymeric compositions are being used for the fabrication of fascia structures. In the one, disclosed in U.S. Pat. No. 3,915,928, the composition comprises an injection moldable mixture of a crystalline copolymer of ethylene and propylene or a terpolymer of ethylene, propylene and a non-conjugated diene; carbon, from 5 to 30 weight percent of chopped glass fibers and sulfur based vulcanizing agents. Parts injection molded from this composition show "trail" lines due to alignment of the glass fibers and the parts must be routinely sanded prior to painting in order to produce acceptable automotive fascia.
In the other, in which the end product is a microcellular polyurethane, described in a paper by Prepelka and Wharton, "Reaction Injection Molding in the Automotive Industry," Journal of Cellular Plastics, p. 87, March/April 1975, the cost of the components comprising the composition is higher than the cost of hydrocarbon based elastomers and production of the polyurethane structures requires specialized metering and mixing equipment and presses.
A variety of compositions and processes have been described in the patent literature for the post vulcanization of preformed elastomers.
U.S. Pat. No. 3,198,868 discloses a process for vulcanizing preshaped articles formed from vulcanizing materials comprising blends of amorphous copolymers of ethylene and higher .alpha.-olefins with polyethylene wherein the shaped article is impregnated with a solution of an organic peroxide, dried and vulcanized by the application of heat.
U.S. Pat. No. 3,256,366 discloses a process for the preparation and vulcanization of a mixture of a copolymer of ethylene and propylene containing 40 to 60 mol percent of ethylene (30.8 to 50.0 weight percent of ethylene) with either low density or high density polyethylene comprising: mixing the polymers at a temperature above 125.degree. C.; adding a peroxide to the mixture at a temperature in the range of about 60.degree. C. to about 90.degree. C.; shaping the mixture to a conformation; and heating the shaped article to vulcanize it at a temperature in the range of about 150.degree. C. to 180.degree. C.
British Pat. No. 1,294,665 discloses cross-linked, electrically conductive, heat-shrinkable polymer compositions having volume resistivities below 1000 ohms-centimeter which comprise mixtures of: at least 40 parts of an electrically conductive filler; at least 20 parts of a natural or synthetic rubber; and at least 10 parts of a normally solid, heat-flowable homo- or copolymer of ethylene.